I. Field of the Invention
The invention relates to a product densification apparatus and system for producing densified feed products, such as sinking aquatic feeds, densified cattle range cubes, rodenticide pellets and the like. The system includes a cooking extruder which is designed to cook and gelatinize cereal grain and other feed formulations. The cooked feed formulations are then passed through a cooking extruder die assembly to yield an expanded feed product. The cooked and expanded feed product is then placed upon a cooling conveyor where it is transported to a specialized extruder called a product densification unit or "PDU". The PDU compresses and densifies the feed product through the use of a specially engineered barrel, die components and a high speed compression screw.
II. Description of the Related Art
Recent interest in the consumption of fish and shellfish due to their high protein and low fat content has led to the development of large scale aquatic fish farming. Catfish, shrimp, crayfish, oysters, and other seafood are now being raised using aquatic farming techniques.
While many bottom-feeding fish, such as catfish, for example, have been successfully conditioned to feed on floating fish food, this option is not available to shrimp and crayfish operations, among others. In these aquatic farming applications, sinking fish food must be utilized.
Sinking fish food has traditionally been produced by feed pellet mill operators as simply another feed product to increase their product line and provide access to additional markets with little additional investment. However, the production of sinking fish foods by traditional pellet mills involves a number of problems. Due to the limited amount of feed preconditioning and the low mechanical shear experienced by feed processed in a pellet mill, the gelatinization of starches is quite low. This makes it difficult for processed feed to properly bind as it leaves the pellet mill. This has forced manufacturers to add artificial binders to the feed stock in order to produce a sinking pellet with good water stability. Many of these artificial binders are polymer based and, therefore, are not biodegradable in water. The resultant water pollution problems have caused some countries to ban the use of these artificial binders. In addition, the relatively low gelatinization rate achieved by pellet mills has adversely affected the nutritional value of the feed. Furthermore, the amount of fat which can be introduced into the raw material prior to its introduction into a pellet mill is very limited. Lastly, it has been difficult to achieve consistent density in feed produced by pellet mills, with some "sinking" feeds produced in such mills actually floating instead.
Another widely used method of producing sinking feed is extrusion cooking. Extrusion cookers are extremely versatile. In extrusion cooking operations, a preconditioner allows the injection and uniform mixing of steam, water, fats or meat slurries, etc. into the cereal grain stock prior to its introduction into the extrusion cooker itself. The extrusion cooker uses mechanical shear, direct steam injection and pressure to cook the premixed raw materials until the desired product characteristics are obtained. Extrusion cooking is commonly used for various types of human and land animal feeds as well as floating aquatic feed. Extruder barrels and other components can be manipulated to produce a wide variety of finished feed product characteristics.
However, in producing sinking aquatic feed via cooking extrusion techniques, a specialized venting barrel segment must be added to the extruder to release the internal pressure built up during cooking. The feed product is then densified after the vented barrel segment by forcing it through a die at the end of the extruder barrel in order to achieve the density necessary for a sinking aquatic feed. Such a vented system is described in U.S. Pat. No. 4,981,711 to Kearns et al. and entitled TEXTURIZED SINKING FOOD FOR MARINE LIFE.
While achieving significantly better results than are available using traditional pellet mills, vented cooking extruders also have a number of disadvantages. Most cooking extruder operators typically produce expanded feed products, such as pet foods and floating aquatic feed, with bulk densities ranging from 24-27 lbs./ft. By contrast, sinking aquatic feed must have a bulk density of between 35-45 lbs./ft. Thus, to produce a sinking feed product, a manufacturer must make a time and labor consuming reconfiguration of the cooking extruder to add the venting barrel section, new screw components and the special die assembly. The resulting vented barrel design also limits the types of materials which can be processed into sinking feed. If high levels of gelatinization are not achieved, the raw materials will not bind properly, causing the product to discharge through the vented barrel section instead of proceeding past to the die assembly. Furthermore, when making small diameter sinking aquatic products, such as shrimp feed, die orifice sizes as small as 1.5 mm are required. When particles larger than the die orifice size reach the die, these die orifices begin to plug up, decreasing the available die open area. This can cause the product to expand, thus decreasing its bulk density. Should the bulk density become light enough to float, the extruder must be shut down so that the die orifices can be cleaned. Also, if the bulk density of the extruded product is borderline, it can be become lighter due to expansion when it is processed through a subsequent dryer/cooler unit. All of these problems result in a production rate of sinking aquatic feed for vented extrusion cookers which is considerably less than that for traditional floating feed and other expanded feed products. In addition, the demanding requirements of sinking feed production requires that an extruder operator pay considerable attention at all times to the raw material grind, product bulk density and production rate. This makes the cooking extruder an extremely operator dependent system when producing sinking feed products.
Similar problems exist in the production of any densified feed product. For example, certain animal feeds, such as cattle "range cubes" are designed to be dropped in a field or cattle feed lot where they may lie for a considerable period of time before they are consumed. Such range cubes must be very dense in order to remain intact during extended exposure to weather. Rodenticides are another example of a feed product, albeit with poison added, which must preferably be made very dense to obtain a long shelf life, remaining intact until they are consumed by a targeted rodent.
It is clear then, that the ever increasing demand for reliable, sinking aquatic feeds and other densified feed products, and the production problems inherent in existing pellet mill and vented cooking extruder systems have resulted in a need for a reliable densified feed production system. Such a system should achieve a relatively high product throughput rate while producing a uniformly dense and highly gelatinized feed product. The resulting system should not require constant monitoring of numerous critical system parameters by an operator.